Several laboratories using focal and diffuse models of traumatic brain injury (TBI) have reported the progressive nature of histopathological damage that can continue in rodent models up to one year. Evidence for progressive damage has previously been reported in humans following head trauma. Preliminary data in this application presented for the first time provides quantitative data for chronic white matter pathology following moderate fluid-percussion brain injury. The overall goal of the proposed studies is to assess the importance of moderate and severe TBI on long-term vulnerability patterns after trauma. In Specific Aim 1, regional patterns of gray and white matter pathology and their associated neurobehavioral alterations will be assessed. Histopathological techniques along with magnetic resonance imaging (MRI) strategies will be used to assess temporal and regional patterns of progressive damage in order to correlate these changes with behavioral outcomes. In Specific Aim 2, immunocytochemical markers of axonal damage, demyelination, cell atrophy, and cell death will be utilized to assess cellular injury progression. To begin to determine the pathomechanisms underlying progressive white matter injury, the role of prolonged hypoperfusion and abnormal protein aggregation on these structural changes will be regionally and temporally assessed. Finally, therapeutic strategies targeting excitotoxic processes as well as abnormal protein aggregation will be evaluated to reduce progression of advancing atrophy. It is felt that these experiments are necessary to understand the pathogenesis of progressive injury as well as to develop therapeutic strategies to promote recovery of function following TBI. Established animal models and behavioral, MRI, immunocytochemical, autoradiography, and molecular techniques will be utilized.